Attenuated total reflection (also referred to as attenuated total reflectance, abbreviated ATR) is a spectroscopic technique, whereby a sample is investigated by means of internal reflection. Electromagnetic radiation, for example radiation in the mid-infrared range, is coupled into an ATR body through a first cut surface and then reflected on a first surface. The radiation is reflected several times along the ATR body between said first and a second surface, wherein each reflection is an attenuated total reflection. One advantage of ATR spectroscopy is that already a very short optical path length is sufficient for the determination of a spectrum. The penetration depth of the radiation into the sample depends on the respective refractive indices of the sample and of the ATR body and on the angle of incidence at which the radiation meets the sample.
If a sample or a measurement medium is brought into contact with one of these surfaces, a part of the radiation is selectively absorbed by the sample at each reflection. The remaining radiation is coupled out of the ATR body at the other end, and the energy distribution of the radiation absorbed by the sample or measurement medium is registered by a detector. The energy distribution can be presented at the output as a function of the wavelength, i.e. as a spectrum, or the absorption is registered only for at least one wavelength. The known state of the art includes bodies of various shapes for the internal reflection. In many cases, the bodies are crystals that are optically transparent for the radiation that is used for the measurement.
ATR spectroscopy is used primarily in the laboratory for the investigation of samples of different kinds. A conventional ATR spectrometer has, besides the ATR body, a radiation source, a detector as well as suitable optical means for coupling the radiation into or out of the ATR body. The sample under investigation can be solid, liquid, or also gaseous.
Besides laboratory applications, other known uses include applications in chemical, biological or physical processes where for example by means of an ATR probe, a measurement medium is investigated which is brought into contact with a measuring surface of the ATR body. Especially in process applications it is often the case that not the entire spectrum is analyzed but that only selected wavelengths are coupled into the ATR body and/or detected, as a way to determine the absorption of specifically targeted substances that are present in the measurement medium and, in addition, to determine their concentration in the process- or measurement medium.
The determination of one or more dissolved substances in a medium, especially the determination of its content proportion or concentration is performed in the most diverse fields such as for example the beverage industry or the field of biotechnology. Examples of such substances are carbon dioxide (CO2), methanol, ethanol, methane, as well as other chemical substances that are contained in a fluid process- or measurement medium, for example in an aqueous solution or the like. Most of all, an accurate knowledge of the CO2 content is of interest for the beverage industry as a part of production control.
ATR sensors with different ATR bodies for in-line measurements are known, for example, from U.S. Pat. No. 7,339,657, which discloses in particular ATR bodies with different geometrical shapes or with a recess on the side facing the measurement medium, so that the measurement medium can be examined with a combination of ATR-and transmission spectroscopy.
A known problem in spectroscopic examinations under process conditions presents itself in calibrating the sensor and in particular checking the calibration, and in performing the initial calibration and/or the recalibration of a sensor that is installed in a process system.
The relationship between the absorption and the concentration of a substance is established through Lambert-Beer's law which, for small concentrations, expresses a linear relationship between the two quantities.
However, if an individual substance or class of substances is to be determined in a complex measurement medium, the problem presents itself that frequently the radiation introduced into the ATR body is absorbed not only by the substance dissolved in the measurement medium but also by the measurement medium itself. Effects of this kind, among others, are referred to as matrix effects.
To minimize matrix effects, the calibration of a process-capable ATR sensor includes not only the measurement of different pure substances but in particular the concentration-dependent measurement of such substances in a measurement medium which may in some cases change its composition. Especially with measurement media of changing composition, for example media of the kind that are present during a beer-brewing process, it would be necessary for a full calibration that all combinations of measurement medium and substance content be measured and taken into account in the calibration, which is most of all very time-consuming.
A calibration of this kind is therefore often performed outside of the process with standardized samples and reflects the actual process conditions only with a certain error tolerance.
To ensure the measurement tolerances and/or the correct functioning of ATR sensors, in particular those which are installed in a process system and are used for the examination of measurement media which are of the same kind or are subject to change, it would be advantageous to have the capability to determine and/or verify the calibration of the sensor in the installed or in-line condition.